What is the correct behavior of: Mail "Reply All"?

Recently I ran into the following problem:

We are a group of people. One and a while we are using email and "Reply All" to have an conversation.

The first mail (say from UserA) has all the address in the "To: field". (UserB, UserC, ...).

But depending of the mail program of the recipient the following happens when selecting "Reply All"

• Apple Mail: The "To field" no contains UserA. All the other addresses have moved into the "Cc: field".
• Other Mail ( i.e. Outlook): All addresses stay in the "To: field"

What is the correct behavior?

Is there an RFC which defines the correct behavior?

Is the behavior not clearly defined and and it is up to the mailer implementation?

Thanks for any hint.